The present invention relates to a process for the production of an extremely selective ion-permeable metal oxide layer on a substrate, a vacuum treatment apparatus for carrying out the stated process as well as a part coated with at least one metal oxide layer. In this disclosure the term "metal oxide" should also be understood to include the oxides of semiconductors.
It is known that metal oxide layers of this type are used in many applications. Without claiming completeness they are used:
as dielectrics for the electronic insulation between conducting layers, PA1 as dielectrics in thin-film capacitors, PA1 as protective layer for encapsulating for example integrated circuits, PA1 as insulators between superconductors for the production of fast switches according to the Josephson effect (Rosenberg, "The Solid State", Oxford University Press, 1988), PA1 for layer structures in solar cells, PA1 for layer structures in optical components, PA1 as gas sensors because of their properties as solid-state electrolytes, PA1 on electrochemical half cells for measuring electromotive force, in particular also for measuring pH, therein also for ion-sensitive semiconductor components, such as GATE layers for field effect transistors, so-called ISFETs. PA1 a high vacuum range or HV is between 10.sup.-1 and 10.sup.-6 mbars, and PA1 an ultra high vacuum range or UHV is pressures below 10.sup.-6 mbars. PA1 at k'/k=1: the oxidation is saturated; PA1 at k'/k&gt;1: the oxidation is supersaturated, and PA1 at k'/k&lt;1: the oxidation is incomplete.
In all of these applications an essential requirement made of the layer is that no microcracks are present in the layer. Microcracks of this type can abolish the function of the oxide layer in all cases. For example, when using the layer as pH sensor layer, microcracks of this type shortcircuit the layer.
A further essential requirement in all stated applications is that of good adhesion to the base, in particular also under the aspect of thermal stresses as well as absence of interference sites or inclusions of foreign bodies.